Wow no.
You have categories, each category can access or not a zone.
You have 31 categories and 31 zones under each category.
You assign numbers or prefixes better said, to a zone.
Your users should be grouped up, 31 groups.
Then you normally allow them to access a specific zone or not... International, local, emergency, etc.
Imagine the headache to manage 15000 users in the way you describe...

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Ignorance is not the problem, the problem is the one who doesn't want to learn